This invention relates generally to fiber optic waveguides and particularly to an absorbing jacket for use with fiber optic waveguides to remove cladding modes therefrom. Still more particularly, this invention relates an absorbing jacket for use with form birefringent optical fibers for propagating a single mode of optical energy in a defined polarization.
It is well-known that optical fibers used for guiding electromagnetic energy in the optical region of the electromagnetic spectrum have a central core that is surrounded by an outer cladding. The index of refraction of the cladding is less than that of the core. Therefore, light propagating in the core sees a dielectric interface at the core-cladding boundary where the refractive index outside the core is less than the core index. Light readily propagates from a material having a low index of refraction to a material having a higher index of refraction. However, when light propagating in a first dielectric impinges upon a dielectric interface where the refractive index decreases as the light crosses the interface, a significant portion of the light may be reflected at the interface back into first dielectric in a classical ray optics model, the internal reflection will be 100% if the angle of incidence is greater than a certain critical angle, which depends upon the ratio of the refractive indices of the materials on the two sides of the interface. The core of an optical fiber is capable of guiding an optical signal because the diameter of the core is so small that light impinges upon the core/cladding interface at angles of incidence that are greater than the critical angle for internal reflection. Therefore, essentially all of a light beam guided by the fiber is in the core; but the boundary conditions at the core/cladding interface require the existence of an exponentially decaying evanescent field that penetrates into the cladding. This evanescent field is correlated with the light guided by the core.
However, it is possible or some small portion of the light to be scattered out of the core into the cladding. Light that propagates in the cladding is generally called a "cladding mode." Cladding modes will degrade the signal guided by core because detectors cannot distinguish the signal from cladding modes. Therefore, cladding modes are undesirable in fiber optic systems, such as rotation sensors and interferometric sensors.
Cladding modes are distinct from the evanescent field of a wave guided by the core and are uncorrelated with information contained in the light guided by the core. Since light easily propagates from a region of lower index of refraction to a region of higher index of refraction, the cladding modes may couple back into the core and remain uncorrelated with the information. If light from the fiber is incident upon a detector both the light carried in the cladding and the portion of the cladding modes that has coupled back into the core are sources of noise, which tends to degrade the integrity of information contained in an optical signal guided by the core.
Conventional single mode and multimode optical fibers are jacketed with a relatively soft plastic material to protect the intrinsically fragile glass fiber from fractures caused by localized applied mechanical stresses. The plastic coating is generally transparent or translucent and has a refractive index greater than the refractive index of the cladding. Therefore, in conventional optical fibers, the cladding modes will eventually radiate into the jacket. However, effective stripping of the cladding modes ordinarily requires tens of meters of fiber after creation of the cladding mode. Excitation of cladding modes is ordinarily to be avoided in conventional optical fibers because the cladding mode represents a loss of signal intensity and is a source of noise in many fiber optic systems and devices.
It is known in the art that encasing the fibers of a bundle in an absorbing glass at a face plate connecting the ends of the fibers to another bundle will alleviate the problem of cross-talk between the fibers.
When materials with different refractive indices are stratified optically and periodically, optical wave behavior is different from that in a uniform medium. In particular, when the thickness of each layer is sufficiently small compared with the light wavelength and the number of layers is sufficiently large, the compound medium is birefringent. Form birefringence results when there is an ordered arrangement of optically isotropic materials (layers) whose size is large compared with the molecules of the materials, but small when compared with the optical wavelength propagating in the fiber. Fiber optic field devices using form birefringent fiber are useful, for example, in constructing gyroscopes, sensors, frequency shifters and communications systems.
Problems arise in using ordinary fibers to form the above listed passive and active components. Strictly speaking, an ordinary axially symmetrical single mode fiber is a "two-mode" fiber because such fibers will propagate two orthogonally polarized He.sub.11 modes. Each polarization has a propagation constant, but an ordinary fiber has propagation constants so nearly equal that degeneracy results. Propagation of two orthogonal polarizations results from the instability of the polarization state of a propagated mode when geometrical perturbations exist in the fiber. Polarization instability degrades the performance of optical fibers in some applications of single-mode fibers in communications and sensing systems.
Previous attempts to provide polarization stability have usually employed one of several methods of maximizing the differences between the propagation constants of the two polarization modes. Elliptical core fibers provide an asymmetrical propagation constant distribution to provide the required difference in propagation constant. Application of an asymmetrical stress distribution by bending a fiber will achieve the same result.
Elliptical core fibers are not practical because producing the desired birefringence in this manner increases the transmission loss to unacceptably high values and because of attendant difficulties in splicing such fibers together and in connecting them to other devices. Stress induced birefringence is subject to relaxation as the fiber optic material flows over extended time periods to relieve the stress. Stressing a fiber to induce birefringence also often results in a fracture of the fiber in the fabrication of devices.